Many metallic pigments are known which provide electrochemical, electrical, thermal, barrier and other properties to compositions which are used for protecting material such as metal from corrosion, maintaining electrical conductivity, shielding equipment from electromagnetic fields, resisting elevated temperatures, and providing protection from moisture. Silver, gold and other noble metal pigments are used for their electrical conductivity and thermal conductivity properties. Zinc and magnesium are used for their electrochemical properties. Aluminum is used for its thermal and chemical barrier properties. A major shortcoming of the noble metals is their strong cathodic potential. When used in products for electrical and thermal management, the noble metals can couple with anodic materials like aluminum alloys used for electrical equipment.
A shortcoming of zinc pigments is low electrical conductivity and low thermal conductivity compared to the noble metals and relatively poor resistance to chlorides and other corrosive materials. A shortcoming of magnesium pigments is high relative anodic potential compared to other metals. In addition, magnesium forms a protective oxide spontaneously in air, rendering it less effective than zinc for the sacrificial protection of steel and other cathodic materials.
Galvanically active metallic powders or “pigments” have been added to coatings or paints for at least 60 years. These pigments are intended to impart galvanic or electrochemical protection to the coated metal, by polarizing the metal to an operating potential which does not allow for corrosion to occur. Coatings containing zinc or magnesium-based pigments are the most common. Zinc is typically used to protect steel. U.S. Pat. No. 3,615,730 discloses zinc-rich primers based on inorganic silica which binds and supports the zinc powder and keeps it intact to the substrate. Zinc-rich primers based on organic epoxy and polyurethane resins are commercially available and specified by DoD in MIL-DTL-24441 Formula 159, MIL-P-21035, and A-A-5975. A major drawback is that zinc-based galvanic primers or protective coatings do not perform well in highly corrosive acidic chloride environments leading to premature corrosion and hydrogen cracking of high-strength steel alloys. Another problem is that zinc is cathodic to aluminum and its alloys in many environmental conditions and therefore will not provide galvanic protection. The relative operating potential of zinc versus aluminum alloys is shown in FIG. 1.
Another class of metal-filled coatings is based on magnesium powder. The US Navy investigated the merit of magnesium-rich epoxy primers and documented the results of performance of these coatings on aluminum alloys in NADC-MA-7063, “Investigation of Metallic Pigments as Corrosion Preventives in Aircraft Primers,” A. Stander, Oct. 5, 1970. These coatings were not adopted by the Navy as they did not perform as well as the chromated epoxy primers qualified to MIL-P-23377, and used on aircraft at the time, especially in SO2 salt fog testing as defined by ASTM G85.
Magnesium-rich primers developed by North Dakota State University and documented in US Patent applications US2007/0128351 A1 and US2009/0155598 A1, which are commercially available from Akzo Nobel exhibit similar performance limitations. Regardless of what is done to these coatings, the operating potential of magnesium remains very high (negative) compared to aluminum, leading to higher self-corrosion rates and lower efficiencies. These coatings also exhibit unusual failure mechanisms which are not well understood.
U.S. Pat. No. 5,336,303 discloses electrochemically active paints based on magnesium, magnesium alloys and calcium alloys which have a high (negative) operating potential to protect steel substrates. These coatings provide protection, but also suffer from high self-corrosion rates, low efficiencies and poor performance in highly acidic chloride environments like those seen by Navy aircraft, ships, and facilities.
Coatings with aluminum powders have been used for over 100 years. These coatings are excellent barriers to the environment and provide good thermal stability and protection. Many bridges, tanks and other steel structures have been painted with aluminum-pigmented coatings over the years with much success. These coatings do not, however, provide galvanic or electrochemical protection of the metal surface on which they are coated, since the aluminum powder or flakes are covered with aluminum oxide which inhibits electrochemical action. These uses and shortcomings are well documented in “Aluminum Paint and Powder by J. D. Edwards and R. I. Wray, 3rd Edition, 1955.
However, magnesium, zinc and aluminum anodes are currently used in bulk form to protect metal from corrosion. To be effective, the anodes need to be in contact electrically with the object they are protecting when immersed in water or an electrolyte. There is extensive literature which describes the pros and cons of each anode material. Aluminum anodes are preferred, since they are inexpensive and provide the highest efficiency of the three metals.
Table 1 shows the electrical out put and cost effectiveness of these three metals based on weight. With regard to recent spot prices for each metal and their relative cost effectiveness clearly aluminum is superior to zinc and magnesium and therefore preferred based on cost, weight and longevity.
Table 1: Comparison of Electrical Properties of Magnesium, Zinc and Aluminum (from Reding, J. T. Newport, J. J.: The Influence of Alloying Elements on Aluminum Anodes in Sea Water. Materials Protection, Vol. 5. December 1966, pages 15-19).
TABLE 1Properties and Costs of Magnesium,Aluminum, and ZincMgAlZnTheoreticalPotential (volts)2.611.901.00ElectricalOutput (amp hrs/lb)10001352372Metal cost (cents/lb)3524.514.5Cost of current at.035.018.039100% efficiency(cents/amp-hr)
Before the 1970's aluminum anodes were not used for the same reasons stated herein for aluminum powders and flakes. The bulk material rapidly passivated, rendering the anode inactive and incapable of protecting the intended metal object. The development of activated aluminum alloys began in the mid-1960's. The intellectual property is documented in U.S. Pat. Nos. 3,379,636; 3,281,239; 3,393,138 by Dow Chemical and U.S. Pat. No. 3,240,688 by Olin Mathesin. All of these alloys were unique in that for the first time bulk aluminum alloys were shown to remain active and galvanically protect metal. Unfortunately, none were commercially successful as they all suffered from low efficiencies making them less economical than zinc anodes. During the 1970's, Dow developed an aluminum anode alloy, which does not passivate and has very high efficiencies, approaching 90% of theoretical.
In addition to being highly efficient versus theoretical, this alloy was designed to have an operating potential of −1.05 volts versus Ag/AgCl reference electrode. This is about the same as zinc and optimum for protecting aluminum and steel structures. Aluminum alloys used on aircraft, amphibious and ground vehicles, and other common DoD and commercial applications have operating potentials that range from about −0.800 to −0.0700 volts and alloy steels have operating potentials around −0.650 volts. The operating potential gap between the aluminum anode and the materials it's designed to protect is therefore about 200-300 millivolts for aluminum and 350 millivolts for steel.
This gap is enough to provide protection, but not so large that the anode rapidly self corrodes, like magnesium. This anode alloy has been investigated for use as a bulk coating of concrete with imbedded steel rebar and applied by flame spray processes.